This invention relates to improvements in aqueous clay-based drilling fluids with better enable drilling at high temperatures by decreasing fluid loss. Aqueous clay-based drilling fluids or muds are well-known in the prior art, as are different additives. Such fluids are comprised basically of water, a clay such as bentonite or sepiolite, lignosulfonate, a base such as NaOH, a densifier such as barite or barium sulfate, and possibly a salt containing a cation such as sodium or potassium. Other aqueous ionic compounds such as Na.sup.+ Cl.sup.- may also be present. These fluids are suitable for drilling at pH levels from about 8 to about 11.5.
As drilling temperatures in recent years have been increasing, the prior art has searched for additives to conventional aqueous or water-based drilling fluids to facilitate drilling at increased temperatures. One major problem some of these additives have tried to alleviate has been an increased fluid loss due to high-temperature drilling. U.S. Pat. No. 4,172,800, issued to Walker, discloses an additive mixture of a polyethoxylated sulfurized fatty acid and a polyalkylene glycol. U.S. Pat. No. 4,201,678, issued to Pye, et al, discloses a foaming solution suitable for drilling at up to 700.degree. F., containing an amphoteric betaine, a salt of a linear aliphatic or alkyl aryl hydrocarbon sulfonate, and, optionally, an unneutralized ammonia. U.S.S.R. Pat. No. 883,140, issued to Boreholes Consolidated, discloses a drilling fluid containing a lignosulfonate, an acrylic polymer, potash alum, potassium hydroxide, and water. This fluid is used at temperatures up to 200.degree. C. and at pH levels from 8 to 12.9. U.S. Pat. No. 4,478,727, issued to Turner, et al, discloses an additive consisting of a co-polymer of sodium styrene sulfonate-co-sodium-N-(4-sulfophenyl)-maleimide.
Other additives which have been used in drilling fluids are vinyl sulfonate amide polymers as described in U.S. Pat. No. 4,309,523 to Engelhardt, et al. Engelhardt discloses polymers consisting essentially of a random distribution of (a) 5 to 95% by weight of units of the formula ##STR4## (b) 5 to 95% by weight of units of the formula ##STR5## and (c) 0 to 80% by weight of units of the formula ##STR6## wherein R.sub.1 and R.sub.2 are the same or different and each is hydrogen, methyl, or ethyl, and X.sup.+ is a cation. This composition has been used in a drilling fluid in U.S. Pat. No. 4,440,649, issued to Loftin, et al. Loftin discloses the use of the terpolymer in the Engelhardt patent as a fluid loss reducing and rheology stabilizing agent, either alone or in combination with pregelatinized starch and/or hydroxyethylcellulose. Loftin also uses organic cationic clay-stabilizing agents as well as water and a viscosity increasing agent. Loftin ages his fluids at temperatures up to 350.degree. F. The fluid loss experiments disclosed in the Loftin reference are standard API fluid loss experiments done at room temperature. This test, as described in Gray and Darley, Composition and Properties of Oil Well Drilling Fluids (1980), pg. 20, subjects the mud to static filtration for 30 minutes, after which the volume of filtrate is measured through filter paper.
U.S. Pat. No. 4,451,631 and West German Offenlegungsschrift No. 3,027,422, issued to Engelhardt, disclose a terpolymer of a random distribution of units of the formulae ##STR7## and partial hydrolysates thereof, wherein when R.sub.1 is hydrogen, R.sub.2 is hydrogen, methyl or ethyl, Y is --CO--NH--C(CH.sub.3).sub.2 --CH.sub.2 -- or a direct bond, and X.sup.+ is a cation; when R.sub.1 and R.sub.2 are identical or different, each is hydrogen, methyl or ethyl or, together, are trimethylene, Y is --CO--NH--C(CH.sub.2).sub.2 --, and X.sup.+ is a cation. U.S. Pat. No. 4,471,097, issued to Uhl, discloses polymers containing a distribution of
(a) 5 to 95 wt.% of units of the formula ##STR8##
(b) 0 to 30 wt.% of units of the formula ##STR9##
(c) 0 to 80 wt.% of units of the formula ##STR10##
(d) 5 to 60 wt.% of units of the formula ##STR11## wherein R.sub.1 and R.sub.2 are the same or different and represent hydrogen, methyl, or ethyl or, together, are trimethylene to cyclic pentamethylene; R.sub.3 is hydrogen or methyl; R.sub.4 is an imidazolyl-(1) or imidazolyl-(2) moiety; Y is a single covalent bond or the formula --CO--NH--C(CH.sub.3).sub.2 --CH.sub.2 --, and X.sup.+ is a cation.
U.S. Pat. Nos. 4,268,400 and 4,309,329, issued to Lucas et al, disclose terpolymers in drilling fluids containing acrylamide and hydroxyalkylacrylate groups, as well as a monovalent alkali metal salt of acrylic acid in the '400 patent or alkali metal acrylate units in the '329 patent. U.S. Pat. No. 4,283,517, issued to Perricone et al, discloses a process for making terpolymers of acrylamide, an alkali metal salt of acrylic acid, and a hydroxyalkyl acrylate. It also discloses a process for making copolymers of (meth)acrylamidoalkylsulfonic acid or alkali metal salt thereof and (meth)acrylamide or N-alkyl(meth)acrylamide. U.S. Pat. No. 4,293,427, issued to Lucas et al, discloses a copolymer of a (meth)acrylamido alkyl sulfonic acid or alkali metal salt thereof and a (meth)acrylamide or N-alkyl(meth)acrylamide. This copolymer can be used in an aqueous drilling fluid which can be aged at temperatures up to 325.degree. F. Lucas also discloses API fluid loss experiments using this drilling fluid at room temperature.
Problems encountered in the prior art include increased fluid loss at high temperatures and/or the decomposition of some drilling fluid additives at high temperatures. The object of the present invention is to alleviate both of these problems with a new additive system for aqueous clay-based drilling fluids.